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fxus65 kcys 181738 aaa 
afdcys

Area forecast discussion...updated
National Weather Service Cheyenne Wyoming
1135 am MDT Tue Jun 18 2019

Update...

See updated 18z aviation weather discussion below.

&&

Short term...(today and tonight)
issued at 306 am MDT Tue Jun 18 2019

High precipitable water airmass remains in place this morning with dewpoints in
the mid to upper 50s across much of southeastern Wyoming and the
Nebraska Panhandle. Showers have died overnight as instability has
wained. Aloft, weak west northwest flow is expected to continue
through this morning and this evening with a series of shortwave
disturbances passing overhead. The combination of shortwave ascent
and high precipitable water air should lead to another round of widespread
showers and thunderstorms. Some marginal severe weather including
large hail and damaging winds will be possible with this activity
through the afternoon hours. Vertical shear remains marginal for
severe weather with ebwd near 25 to 30 kts. Instability is the
bigger question with some models showing much reduced values
compared to yesterday. Hi-res guidance continues to suggest
widespread stratus and fog this morning which is being observed by
some aviation sites all be it not as dense as forecasted. This
fog and cloud cover may retard surface heating through the
afternoon limiting available instability. Hi-res ensembles show
large spread among members suggesting a low confidence forecast.
If cloud cover breaks and sufficient insolation is obtained,
instability on the order of 500-1000 MLCAPE could be expected
given the rich surface moisture and favorable midlevel lapse rates
near 7 to 7.5 c/km. The threat for flash flooding from slow
moving storms will also become greater if instability is realized
and storm coverage is increased. Storms have been efficient rain
producers and the stagnant high precipitable water airmass will not modify much
this afternoon and evening. Any storms that develop will have the
potential for flash flooding. Given the uncertain nature of the
heaviest precip, choose to not go with a Flash Flood Watch due to
concerns about coverage and limited intensity. Conditions will
need to be monitored for a potential increase in severe
probabilities to marginal if greater instability is realized.

Lingering showers will gradually clear from west to east
overnight as winds become westerly and stronger ahead of an
approaching upper level trough. Clouds should give way to mostly
clear conditions with a few high clouds by midnight. Overnight
lows will run 5 or so degrees below normal with lows in the mid
to upper 40s.

Long term...(wednesday through monday)
issued at 306 am MDT Tue Jun 18 2019

Anomalously strong mid level troughing will develop Wednesday as
a shortwave trough digs down across the intermountain west.
Accompanying the trough, will be an unusually strong mid level
jet. A strong Lee low will develop Wednesday across northern and
central Wyoming before ejecting eastward onto the Great Plains.
Temperatures will be significantly warmer with highs in the mid
70s to near 80. A few showers and high based thunderstorms will be
possible Wednesday evening but westerly winds will scour out much
of the available low level moisture. 5 and 700 mb flow will
continue to increase Wednesday afternoon and evening with 500 mb
flow to 75 kts and 700 mb flow to near 50 kts from the west.
Model Omega and pressure fields show a strong low level pressure
couplet developing across the area. Low level gradient winds will
accelerate Wednesday afternoon and evening aided by deep boundary
layer mixing and momentum Transfer. Gap winds could increase to
high wind criteria by Wednesday afternoon/evening and may continue
to gust overnight into Thursday/Thursday afternoon. High wind
events this time of year are rare, and it remains unclear how
this event will play out. The anomalous upper level flow does
signal potential for high winds in gap areas along I-80 and will
need to be watched carefully as details evolve.

Wetter weather is expected to return early Friday as the
shortwave trough ejects to the north and the east setting up
favorable low level flow for moisture return. Showers and
thunderstorms are expected through the weekend and into early next
week. Temperatures will remain below average through this time
period. Upper level troughing will remain in place through the end
of the period signaling a wet and active pattern continuing.

&&

Aviation...(for the 18z tafs through Wednesday morning)
issued at 1130 am MDT Tue Jun 18 2019

Widespread MVFR ceilings are expected, especially for the western
Nebraska terminals and kcys this afternoon. Showers and
thunderstorms are already developing and should continue to spread
east. Kept ts in the forecast for klar and kcys, but thunderstorms
will struggle to develop across western Nebraska through most of the
afternoon due to low cigs, so will add thunderstorms in the vicinity for those terminals
through this evening. IFR conditions are possible with any
thunderstorms may due to visible between 1 to 3 miles. Otherwise,
thunderstorm and shower activity will slowly end between 02z to 07z
tonight, with a chance of some more low clouds and fog after 07z
towards early Wednesday morning.

&&

Fire weather...
issued at 306 am MDT Tue Jun 18 2019

Fire weather concerns will be low for the first part of the
forecast period as wet weather continues. Wednesday and Thursday,
temperatures will warm and winds will increase. Humidity could
drop to near critical values. Recent rainfall should keep
conditions non-critical before more wet weather returns this
weekend.

&&

Hydrology...
issued at 306 am MDT Tue Jun 18 2019

Overall not much change is expected in the flood situation through
the next several days. Precipitation chances will remain likely
for the area through today. A brief break with drier weather is
expected Wednesday and Thursday before wet conditions return this
weekend and next week. High volume flows are expected to continue
at Saratoga and other area rivers and streams. A continued wet
and active period will continue to pose a Hydro threat through the
extended.

..prev outlook...

Hydrologic concerns will continue for the Upper North
Platte and Laramie rivers and their tributaries due to a ripe high
elevation snow pack and increasing chances for rain on snow-melt
today through today. Latest rfc forecasts maintain minor flood
potential near Saratoga and along portions of the Laramie
watershed. Current model quantitative precipitation forecast indicates the potential for
significant precipitation above 9000 feet in the form of rain with
daytime temperatures in the 50s and nighttime low temperatures
around 37 degrees. Runoff due to saturated soils will likely cause
streams and rivers to rise through Tuesday. A river Flood Warning
remains in effect for the Upper North Platte River near Saratoga.
Additional flood watches and warnings may be required with the
potential for heavy slow-moving convection.

&&

Cys watches/warnings/advisories...
Wyoming...none.
NE...none.
&&

$$

Update...jsa

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